Polysilazane green fibers are precursors to silicon carbide (SiC) fibers. These green fibers must be cross-linked to make them infusible so that the fiber's dimensional integrity is maintained during subsequent pyrolysis steps. The current commercially viable crosslinking process of green fibers involves exposure of a package of polycarbosilane or polysilazane green fibers to high energy e-beam radiation. However, the high energy e-beam system is prohibitively expensive due to the large capital investment. Further, the currently practiced irradiation process takes several hours due to the requirement that the temperature of irradiated fibers not reach the melting point of polysilazane resin. The required large e-beam dose, therefore, must be delivered at a slow rate. The irradiated fiber package then must ride around a long conveyor belt to cool down before returning to the e-beam for another small dose. The package goes under the beam enough times to receive the cumulative dose needed for effective crosslinking.
Other prior art methods of making polysilazane green fibers infusible involve exposure of the fibers to moisture, which provides a cross-linked fiber with high level of oxygen. Some specially formulated polysilazane green fibers may also be cross-linked by UV irradiation. Other methods involve exposing the green fibers to reactive and toxic gases such as ammonia, BCl3 or HSiCl3. Such processes present several environmental health and safety challenges and are expensive due to the toxic nature of the reagents.
Boron is used as a sintering aid during high temperature heat treatment of silicon carbide ceramic materials. In general, boron is present at relatively low concentrations. The presence of boron sintering aid at higher concentrations leads to an increase in undesirable high temperature mechanical properties of the ceramic, such as creep.